## Why

The implicit note shorthand (`kb "some text"`) makes it too easy to accidentally add notes when mistyping commands. Despite the single-word guard, any multi-word typo (e.g. `kb lisst --type pdf`) silently creates a note. The root command doing double-duty as both command dispatcher and note ingester undermines user trust. Reverting to explicit, structured add commands eliminates accidental ingestion and gives every content type a clear, discoverable verb.

## What Changes

- **New `addnote` command**: `kb addnote <text>` takes a single quoted positional argument and submits it as a note. Supports `--tags`. The `submitNote()` logic moves from `root.go` to a new `addnote.go` command file.
- **Remove implicit note shorthand**: The root command reverts to standard Cobra behaviour — no `ArbitraryArgs`, no special arg-count logic, no `--tags` flag on root. Unknown input gets Cobra's default "unknown command" error.
- **Remove custom usage template**: The root command no longer needs the `isRootCmd` template logic. Standard Cobra usage template for all commands.
- **Update examples**: `examples.go` updated to show `kb addnote` instead of bare `kb "text"`.
- **Update tests**: Remove implicit note shorthand tests, add `addnote` command tests.
- **`addfile` unchanged**: Stays exactly as-is.
- **BREAKING**: `kb "note text"` no longer works. Users must use `kb addnote "note text"`.

## Capabilities

### New Capabilities

_(none)_

### Modified Capabilities

- `go-client`: The "Implicit note shorthand" requirement is removed entirely and replaced by a new "Add note command" requirement. The "Add command (file and note ingestion)" requirement description is updated to reflect `addnote` / `addfile` as the two ingestion commands. The root command reverts to standard Cobra behaviour with no custom arg handling or usage template.

## Impact

- `client/cmd/root.go` — remove `ArbitraryArgs`, `RunE` note logic, `--tags` flag, custom usage template, `isRootCmd` template func
- `client/cmd/add.go` — `submitNote()` function moves to new `addnote.go` (or stays in `add.go` alongside `addfile` — design decision)
- `client/cmd/addnote.go` — new file defining the `addnote` command
- `client/cmd/examples.go` — update example text
- `client/cmd/root_test.go` — remove implicit note shorthand tests, add standard Cobra behaviour tests
- No engine changes — the API contract (`POST /api/v1/jobs` with `note` field) is unchanged